1. Field of the Invention
The present invention relates to a corona generating apparatus and, more particularly, to a corona generating apparatus having a novel corona charge regulating member.
2. Description of the Prior Art
In general, the image forming process by an electrophotographic copying machine includes the steps of forming, on a charge retaining member having a photosensitive member consisting of an insulating photoconductive material, a latent image corresponding to the original image, by a corona discharge and an irradiation of light, and developing the latent image by a developer consisting mainly of thermoplastic colored fine particles to form a visible image.
In ordinary electrophotographic copying machine, suitable means are provided to regulate the width of charging of the photosensitive member to form peripheral area requiring no visualized image, i.e. non-image forming region, in order to save the toner and to prevent the contamination of peripheral portion of the recording paper by the toner.
The means for regulating the charging width on the photosensitive member, for instance, includes a regulating member made of an insulating material and disposed between the corona wire and the photosensitive member. By the presence of this regulating member, the generation of the corona discharge is avoided and the flow of the corona charge from the corona generating apparatus to the photosensitive member is physically prevented.
This regulating method, however, poses the following problem. For instance, when the discharge is made continuously due to a continuous copying operation, the insulating regulating member is electrostatically charged so that the corona ion falls onto the photosensitive member at positions away from the regulating member, by the force acting between the charge and the corona ion.
In other words, the actual charging width is smaller than the width of the corona generator regulated by the charging regulating member of the corona generating apparatus.
Another drawback is that the charging width tends to fluctuate depending on the state of charging of the insulating regulating member. It is also known that the fluctuation owes also to other factors such as the distance between the regulating member and the photosensitive member, material of the regulating member, continuity of the copying work and the charging voltage. Namely, the fluctuation is increased as the distance between the regulating member and the photosensitive member is increased, and as the insulating power of the regulating member is increased. Similarly, the fluctuation is increased as the charging voltage becomes lower and as the copying operation is continued.
On the other hand, the charging width fluctuates also by the change in the distribution of electric field between the corona wire and the photosensitive member near the end of the regulating member. It is, therefore, necessary to stabilize this electric field. If the regulating member is made of a conductive material, the potential of the regulating member is advantageously kept at 0 volt. In this case, however, the strength of the electric field between the wire and the regulating member is increased excessively so that the regulating member absorbs even the charges which are away from the ends of the regulating member. Consequently, a potential difference is created between the central portion and end portions of the photosensitive member. In some cases, a thundering is caused between the wire and the regulating member. The use of the conductive material as the material of the regulating member, therefore, poses defects in quality of the picture and the safety.
To avoid these defects, it has been proposed to use a semi-conductive material as the material of the regulating member or to apply a bias voltage of the same potential as the photosensitive member to the conductive regulating member. The use of the semi-conductive material, however, poses a difficulty in setting the path for stably flowing the electrostatic charges, as well as unstable factors concerning the control of the semi-conductivity of the material. On the other hand, the regulating method employing a bias voltage is also unsatisfactory because of an increase of the electric current into the biasing circuit resulting in a lowered charging efficiency and raised cost.